Our current understanding of the event dubbed the Big Bang implies that all matter, and all energy, appeared in our Universe at that moment. Ever since then, the Universe has been changing, and those changes are reflected in the behavior of matter and energy.
Unfortunately, we can’t observe this event directly in order to get a better understanding of how the earliest stages of the Universe’s evolution unfolded. This isn’t something that can be fixed with a more powerful telescope, either; at an early point in time, the Universe was quite simply opaque to light. And much in the same way that I can’t see through furniture, even the best observatories can’t penetrate into the depths of time beyond this point.
To explain how we know that dark matter has been around since the very early Universe, I have to talk a bit about how we’ve managed to learn about the early Universe in general.
You may have seen this image before. The cosmic microwave background, which has the alternate name of the “oldest light in the Universe” is the surface of the cloudy Universe, just as it became transparent to light. There’s a lot of information about the early Universe embedded in this image, which tells us about the temperature of the Universe, and how the matter which later became clusters of galaxies was distributed around.
It’s this latter point which points to the existence of dark matter in the very early Universe.
One of the ways this image is analysed is by looking at the characteristic spacing between bright and cool patches. If this were 100% randomly distributed, you wouldn’t expect to find things at any particular distance more frequently than any other distance. And that could have been the way our Universe was arranged, but it isn’t. Our Universe has a few preferred spacings.
One of these sets of spacings is formally termed Baryon Acoustic Oscillations. Baryons are simply any matter, the protons and neutrons and electrons that make up humans and stars and dogs. Acoustic oscillations should sound like a musical term more than a physics one, and it’s not without reason. The early Universe was much like a fluid in many ways, and information travelled through it like sound through water. The Baryon Acoustic Oscillations, therefore, refers to the way that the stuff of atoms rippled through the early Universe.
The cause of those ripples? A pressure from dark matter. When the Universe became less fluid, the protons, electrons and neutrons were frozen into place, wherever their ripple had left them. There’s a huge number of these ripples in the Universe, so rather than a single stone thrown into a lake, our image is more like a water surface pummeled by a tropical rainstorm. The spacing between where the dark matter sat, and where some of the baryons in our Universe had rippled outwards to, is reflected in the statistics of the cosmic microwave image. Without the presence of dark matter, we wouldn’t expect to see this exact spacing. And if dark matter was present so early in the Universe, it probably was formed in the big bang. At the very least, it couldn’t have been formed later, the way the heavy elements in our Universe were formed by stars, many years after this snapshot of our early Universe.
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